The present invention relates to a multi-output power supply, comprising multiple switching power supplies provided in parallel, for supplying powers having various specifications to multiple loads, and to an electronic device using the multiple switching power supply.
In recent years, for the purpose of meeting the needs for energy conservation, some electronic devices are configured that the operations of the electronic circuits provided therein are temporarily suppressed or stopped according to the working environment and working conditions thereof. Furthermore, recent electronic devices are ingeniously designed to reduce power consumption as much as possible by optimally changing the voltage of the power supply thereof.
Moreover, although conventional electronic devices are each configured to drive multiple load circuits using one power supply voltage, recent electronic devices are each required to generate different power supply voltages so that optimum power supply voltages are supplied to the respective multiple load circuits thereof. Hence, for the purpose of meeting such a need, a conventional power supply is configured to comprise multiple switching power supplies provided in parallel so that individually-controlled powers are output to respective load circuits. Such a multi-output switching power supply is required to comprise devices having quick response to changes in the output specifications in each load circuit. For the purpose of responding to such changes in the output specifications, a control method wherein not only the output voltage but also the output current of a step-down switching power supply is detected to respond to abrupt changes in load has been disclosed in the publication of examined Japanese Utility Model Application, Publication No. Hei 8-11055, for example. The operation of the step-down switching power supply described in the Japanese Utility Model Publication No. Hei 8-11055 will be described below using FIG. 11.
FIG. 11 shows the configuration of the conventional step-down switching power supply, and is a block diagram obtained by simplifying the step-down switching power supply shown in FIG. 1 of the Japanese Utility Model Publication No. Hei 8-11055. However, for convenience in description, the names and numerals of the components shown in FIG. 1 of the Japanese Utility Model Publication No. Hei 8-11055 are changed in the step-down switching power supply shown in FIG. 11. In FIG. 11, numeral 101 designates an input power supply, and the input power supply 101 outputs an input DC voltage Vi to a converter section 102. The converter section 102 comprises a switch 111, an inductor 112, a rectifier 113 and an output capacitor 114. Furthermore, the converter section 102 is driven and controlled by a control section 115 and supplies a power to a load 103.
In the converter section 102, one terminal of the switch 111 is connected to the input power supply 101. The switch 111 is turned ON or OFF by a drive signal V111 output from the control section 115. The other terminal of the switch 111 is connected to one terminal of the inductor 112 and the output terminal of the rectifier 113. The other terminal of the inductor 112 is connected to the output capacitor 114. An output DC voltage Vo is supplied from the output capacitor 114 to the load 103. When the switch 111 is ON, current flows from the input power supply 101 through the switch 111 and the inductor 112 to the output capacitor 114 and the load 103, returning to the input power supply 101. Hence, magnetic energy is stored in the inductor 112, and a power is supplied to the load 103. When the switch 111 is OFF, current flows from the rectifier 113 through the inductor 112 to the output capacitor 114 and the load 103, returning to the rectifier 113. Hence, the magnetic energy of the inductor 112 is released, and a power is supplied to the load 103.
A stable power is thus supplied to the load 103 by the periodic repetition of the above-mentioned ON/OFF operation of the switch 111. When it is assumed that the ratio of the ON time in one switching cycle of the switch 111 is duty ratio δ, the relationship between the input DC voltage Vi and the output DC voltage Vo is represented by the following expression (1).Vo=δ·Vi  (1)
Therefore, the output DC voltage Vo can be controlled by adjusting the duty ratio δ of the switch 111. In the expression (1), it is assumed that the voltage drop at the time when each of the components, such as the switch 111, is ON is negligible.
In FIG. 11, the control section 115 comprises a detection circuit 116, an oscillation circuit 117, a transient response adjustment circuit 118, an adder 119 and a pulse-width modulation circuit 120. The detection circuit 116 detects the output DC voltage Vo and outputs a control signal Ve corresponding to the output DC voltage Vo. The control signal Ve lowers as the output DC voltage Vo is higher than its target value. Conversely, the control signal Ve rises as the output DC voltage Vo is lower than its target value. The oscillation circuit 117 outputs a triangular wave signal Vt that increases/decreases at a predetermined frequency. The transient response adjustment circuit 118 detects the output current Io of the converter section 102, compares the output current Io with an output setting current Iset, and outputs a correction signal Va. The correction signal Va is added to the control signal Ve at the adder 119, and the resulting signal is input to the pulse-width modulation circuit 120. The pulse-width modulation circuit 120 compares the sum signal (Ve+Va) of the control signal Ve and the correction signal Va with the triangular wave signal Vt, and outputs the drive signal V111. When the sum signal (Ve+Va) is higher than the triangular wave signal Vt, the drive signal V111 becomes H level, thereby turning ON the switch 111. When the sum signal (Ve+Va) is lower than the triangular wave signal Vt, the drive signal V111 becomes L level, thereby turning OFF the switch 111.
A configuration wherein the transient response adjustment circuit 118 and the adder 119 are eliminated from the step-down switching power supply configured as described above so that the control signal Ve is directly input to the pulse-width modulation circuit 120 is taken as a conventional example in Japanese Utility Model Publication No. Hei 8-11055. The control system being used in this conventional example is a general control system referred to as the voltage mode control system. Even in the circuit configuration shown in FIG. 11, during ordinary operation wherein the transient response adjustment circuit 118 does not operate, that is, in the case of Va=0, the operation of the circuit configuration is the operation conforming to the conventional voltage mode control system being used generally.
FIG. 12 is a waveform diagram showing the operations of the various sections of the control section 115. In FIG. 12, (a) shows the control signal Ve, (b) shows the correction signal Va, (c) shows the triangular wave signal Vt and the sum signal (Ve+Va), and (d) shows the drive signal V111. The operation states in the left half portion of the waveform diagram of FIG. 12, that is, the operation states up to time t5, correspond to the ordinary operation wherein the transient response adjustment circuit 118 does not operate, that is, the correction signal Va is zero. At time t0, the triangular wave signal Vt begins to lower. At this time, the triangular wave signal Vt is higher than the control signal Ve, the drive signal V111 is L level, and the switch 111 is OFF. When the triangular wave signal Vt becomes lower than the control signal Ve at time t1, the drive signal V111 becomes H level, and the switch 111 is turned ON. The triangular wave signal Vt stops lowering and begins to rise at time t2. When the triangular wave signal Vt becomes higher than the control signal Ve at time t3, the drive signal V111 becomes L level, and the switch 111 is turned OFF. At time t4, the triangular wave signal Vt stops rising and begins to lower. After time t4, an operation similar to that carried out after time to is repeated. When the output DC voltage Vo becomes higher than its target value, the control signal Ve lowers, and the period during which the control signal Ve is higher than the triangular wave signal Vt is shortened. In other words, the ON time during which the drive signal V111 is H level and the switch 111 is ON is shortened. As a result, the output DC voltage Vo having been higher than its target value lowers.
Conversely, when the output DC voltage Vo becomes lower than its target value, the control signal Ve rises, and the ON time is lengthened. As a result, the output DC voltage Vo having been lower than its target value rises. In this way, the output DC voltage Vo is stabilized to its target value.
Next, it is assumed that the load 103 becomes heavy abruptly and the output current Io increases abruptly at time t5. The detection circuit 116 detects the drop of the output DC voltage Vo owing to the abrupt increase of the load, and the control signal Ve rises. On the other hand, the transient response adjustment circuit 118, being used to detect the output current Io, detects the abrupt increase of the load earlier than the detection circuit 116, and raises the correction signal Va before the control signal Ve rises. Since the pulse-width modulation circuit 120 compares the sum signal (Ve+Va) of the control signal Ve and the correction signal Va with the triangular wave signal Vt, the H level period of the drive signal V111, that is, the ON time of the switch 111, is lengthened instantaneously.
The conventional step-down switching power supply configured as described above is capable of preventing the output DC voltage Vo from lowering by instantaneously responding to abrupt increase in load.
As described above, in an electronic device, for the purpose of supplying power supply voltages best suited for respective multiple load circuits, switching power supplies generating different power supply voltages are required to be provided in parallel. However, in the case that the multiple conventional switching power supplies described above are provided in parallel to output powers controlled individually as described above to the respective load circuits, the size of the circuit becomes large, thereby causing a problem of making the device large. When the respective switching power supplies carry out switching operation at the frequencies of the oscillation circuits respectively incorporated therein, there is a problem of generating abnormal noise or increasing an output ripple voltage owing to a low-frequency beat phenomenon caused by adjacent switching frequencies. Furthermore, the generation of multiple noise spectra owing to the multiple switching frequencies causes other devices and circuits to malfunction, and causes a problem of making countermeasures against noise difficult. Moreover, although the conventional switching power supply configured as described above can respond to abrupt changes in load by detecting the output current and by adding the transient response adjustment circuit, the conventional switching power supply has a problem of being unable to respond to changes in the output DC voltage.